Tip a base layer into a low tub or tray. River pebbles from the garden centre or hardware store work beautifully. They are usually sold cheaply for paving and drainage and they have that cool smooth weight in the hand. Just the best! You can add a second base like white rice or yellow split peas or black beans if you want contrast under the stones. The sounds are glorious and matter here as much as the look.
Drop in a wooden treasure box. Scatter a few jewels, old keys, crystals, coins, magnetic chips with a wand, anything that reads treasure. Add scoops, small containers, funnels, ladles. The more it feels like real adventure gear, the deeper they go!
For toddlers or any group where mouthing is likely, swap tiny loose parts for safer larger pieces. Dried citrus slices, large shells, big wooden discs. You still get the ritual of finding and collecting without the choke risk.